Radiant electric heaters are known in which one or more heating elements are supported on or above a layer of thermal insulation material compacted in a metal support dish. Heaters of this kind are described, for example, in GB-A-1 580 909 (in which the heating element comprises coiled bare electric resistance wire) and EP-A-0 117 346 and GB-A-2 146 431 (in which the heating element comprises one or more infra-red lamps). Such heaters are typically incorporated in cookers, cooktops and other heating appliances having a flat, glass-ceramic cooking/heating surface.
Different power levels are provided, in one known technique, by including two or more heating elements in the heater, with a switch arranged to couple the elements into different series and/or parallel configurations in different switch positions.
In designing such heaters various different and possibly conflicting requirements must be accommodated. The resistances of the different heating elements must be chosen so that differing combinations of the elements produce power levels which together form an appropriately distributed sequence between zero and full power. In particular it has been found generally desirable to include a very low power level, of the order of 5% of full power. Where possible the intensity of visible light radiation produced by the energized elements at each power setting should be indicative of the power level at that setting; in addition it may be desirable that one or more elements should be energized sufficiently to produce some visible radiation for as many power settings as possible, to provide assurance to the user that the heater is functioning. This is particularly the case for any infra-red lamp heating element that may be included in the heater; the presence of an infra-red lamp element in an appliance typically results a premium price, so the user is likely to expect that element to be visibly in use.
The values of resistance that are chosen for the elements to suit these requirements will (in the case of coiled resistance wire elements) involve corresponding lengths (typically of the order of several meters) of resistance wire to provide those resistance values. These lengths of wire must be accommodated in the relatively confined space of the heater unit in such a way that electrical insulation constraints are satisfied, and so that an appropriate distribution of heat is obtained for each power level setting together with an aesthetically pleasing appearance.
In the case of heaters with infra-red lamps there is an additional constraint of limiting the number of lamps required, in view of their relatively high cost. This in turn limits the number of different series/parallel configurations in which the lamps can be connected.
Another way of providing switched power levels is possible in those countries, such as Germany and USA, where domestic electricity supplies typically comprise more than one phase of a multi-phase power distribution system. In this case it is possible to connect a heating element either between the line for one supply phase and neutral to obtain a first power level, or between the lines for two different phases to obtain a second, higher power level. An arrangement of this kind is described in U.S. Pat. No. 2,900,480.
Arrangements for switching the power level of radiant heaters currently provide a total of six different power levels plus off, using for example three heating elements. Nonetheless, it is believed that a need exists for switching arrangements providing a larger number of user-selectable power levels. However simple extension of the techniques already in use is not feasible, because for example the required number of heating elements could not practically be accommodated.